1. Field of the Invention
The present invention relates to an image-forming optical system used for a projection type image display apparatus such as a projector and an image pickup apparatus such as a camera.
2. Description of the Related Art
A projection type image display apparatus displays an image by illuminating an image display element such as a liquid crystal panel or digital micro mirror device with luminous flux from a light source and enlarging/projecting transmitted light or reflected light modulated by the image display element on a screen, etc. using a projection lens.
FIG. 4 shows a projection optical system disclosed in International Publication No. WO97/01787, which relates to patents to be republished. In this optical system, luminous flux emitted from a light source 101a is reflected by reflection mirrors such as illumination optical systems 101b, 101c and 101d and incident upon a reflection type image display panel 102. Then, the luminous flux modulated and reflected by the image display panel 102 is reflected by mirrors 103a, 103b, 103d, 103e and a flat mirror 103f which are capable of image-forming, and diagonally projected onto a screen 104.
On the other hand, various image-forming optical systems using a decentered optical system aiming at miniaturization of the entire optical system have been recently proposed. A decentered optical system introduces a concept called a “reference optical axis” and can construct an optical system with aberration sufficiently corrected by forming a rotationally asymmetric aspherical surface or so-called free form surface. For example, Japanese Patent Laid-Open No. 9-5650 proposes the design method and Japanese Patent Laid-Open No. 8-292371 and Japanese Patent Laid-Open No. 8-292372 propose design examples thereof.
When an off-axial optical system, that is, a reference optical axis along the ray penetrating the center of an object (or the center of an image) and the center of the pupil is considered, this decentered optical system is defined as an optical system including an off-axial curved surface where the plane normal at the intersection with the reference optical axis of the configured surface is not on the optical axis and is referred to as an optical system with a folded reference optical axis.
This optical system, by appropriately configuring, prevents eclipse even on the reflecting surfaces, and therefore it is easier to construct an optical system using the reflecting surfaces. The off-axial optical system also features the ability to route optical paths relatively freely. Furthermore, using a reflection image-forming optical system only using surface curved mirrors makes it possible to remove almost all influences of chromatic aberration which is a problem of a refractor system.
In a projection optical system disclosed in International Publication No. WO97/01787 shown in FIG. 4, reflectors 103a, 103b, 103d and 103e having image-forming action in particular are constructed of rotationally symmetric aspherical reflectors having a common rotation axis and images are diagonally projected using the reflectors of these concave mirrors and convex mirrors partially.
However, there are restrictions on the degree of freedom, such that the surfaces should have a common axis, and therefore there are limitations to correcting aberration and brightening the reflection optical system (reducing the F number).
Furthermore, according to this projection optical system, luminous flux that has passed through an aperture-stop 103c is incident upon a convex mirror 103b and the divergent luminous flux from this convex mirror 103b is incident upon the next convex mirror 103d. For this reason, the effective diameter of the convex mirror 103d has a tendency to increase.
In this way, the distance between the reflectors of the projection image-forming optical system constructed by combining a plurality of mirrors tends to increase and the problem is that the size of the entire apparatus increases.
With regard to an image projection apparatus, a projection apparatus generally uses a transmission type liquid crystals for the image display panel. Furthermore, as an image-forming optical system used for the projection apparatus, almost all products use refraction lenses under actual circumstances. In the image-forming optical system used with this transmission type liquid crystal panel device, it is well known that the object, which is the image display panel, needs to have a telecentric optical configuration in order to improve light utilization efficiency.
Though it depends on the specification of the product, the projection image-forming optical system is generally required to be brighter than F3.0 in order to reduce the load on the illumination optical system, reduce costs and power consumption and provide optimal apparatus performance.
When an image-forming optical system constructed by combining a plurality of curved reflection mirrors is used instead of a projection image-forming system using refraction lenses, the off-axial optical system is characterized in that the off-axial optical system can set the projection angle (projection angle of elevation) high (large) more easily than the refraction optical system.
However, designing the refraction optical system with high projection angles requires an extremely wide angle of view, which results in a problem that the design becomes more difficult and the diameter of lenses increases.
Constructing an optical system by only combining surface reflection mirrors with a hollow configuration per se has an advantage of preventing influences of chromatic aberration, etc.
However, attempting to apply an image-forming optical system combining curved reflection mirrors as the projection image-forming optical system meeting requirements of the projection apparatus using the above-described liquid crystal panel involves the following problems.
In the case of a lens, which is brighter than F3.0 on the image display panel side, it is unavoidable that the effective diameter of the first mirror on the object side increases. Furthermore, as described above, since the object side is telecentric, points from which spread luminous flux is emitted in the direction perpendicular to the surface of the object are arranged side by side with the height of the object corresponding to the size of the image display panel as object points, and the effective diameter of the first mirror unavoidably increases all the more. This results in a problem that the size of the entire optical system increases.